Electrically operated devices that function in a cordless mode typically include a housing which has a chamber for receiving and retaining a removable battery module. The battery module completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recently, improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. A hand-held power tool is typically powered by a battery module that comprises a number of batteries connected in series. To provide the higher power levels required by high power devices an increased number of batteries are connected in series resulting in higher input voltages and battery module volumetric requirements.
Cordless power devices permit work operations to be performed in areas where a conventional AC power source is not available or inconvenient to use. However, the additional power interface that is required to allow battery modules to be removed for replacement and charging leads to a decrease in the reliability of the power tool. Cordless power devices universally employ electrical contacts that are incorporated into terminal blocks and connectors as the interface to electrically couple the battery module to the power device. Over the lifetime of a power device, the electrical contacts are subjected to numerous events that may lead to the eventual wear-out or premature failure of the power interface. The wear-out mechanisms include wearing due to contamination, damage, or misalignment of the terminals, as well as high currents and contact bounce caused by high vibration environments. In addition, material discrepancies and high cross-sectional currents may contribute to wearing of the contacts. As the power interface degrades, the impedance of the connection increases leading to higher power losses in the interface. The higher power losses cause increased localized heating of the contacts that is further exacerbated by the thermally isolated nature of most power interfaces, resulting in a further increase in temperature. The high temperatures contribute to the degradation of the contacts and might eventually lead to thermal runaway, resulting in melting of the connector case. A cordless power device that is not repaired before thermal runaway occurs might be irreparable. Since, a cordless power device receives electrical energy from a limited source, the battery, the device is less likely to suffer thermal runaway than a power device that operates from an unlimited power source such as 115 Vac line power. The limited nature of battery power restricts the quantity of power that might be dissipated in the power interface, thereby limiting the amount of damage to which the interface will be subjected.
There is another class of power devices, dual-mode power devices, that have recently been introduced that have a power interface and an unlimited source of electrical energy. Dual-mode power devices include an optional corded converter module that connects to an AC power source and is designed to be interchangeable with the battery module. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. The converter module allows a power device operator to use the device in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of a converter module enables the operator to complete a project when the battery module has been discharged. However, when the dual-mode device is operated with the converter module, the power interface has the potential for receiving much greater damage under failure conditions due to the unlimited power source. Therefore, it is desirable to disable a degraded power device before extensive damage occurs to the power interface.
While the prior art can be used to provide cordless and dual-mode power devices, it has not proven capable of minimizing the potential failure of the associated power interface for the power devices.